1. Field of the Invention
The present invention relates to non-stick ceramic coating compositions and methods for making same, and for coating utensils or cookware made of stainless steel, aluminum, aluminum alloy, cast iron or steel.
2. Prior Art
Considerable effort has been directed at producing an effective, non-stick and non-wetting coating on a substrate that constitutes the surface of cookware for preparing food. The term “non-stick” signifies a surface that resists adherence of substances, such as oils, greases, or other food items. This property permits easy cleaning of dishes in which foods are prepared by heating. The term “non-wetting” indicates a surface that repels liquids such as water. The property is evidenced by a large contact angle between a drop of water and the surface on which the drop rests. An advancing angle of at least 90.degree, is considered as representative of a non-wettable surface.
Non-stick, non-wetting articles are commonly produced by coating article surfaces with a polytetrafluoroethylene (PTFE) layer. For example, U.S. Pat. No. 4,683,168 (Hares et al.) describes a process for coating glass or glass-ceramic articles with such a layer to produce non-stick cooking utensils. However, PTFE coatings exhibit the disadvantage of being opaque. Also, they require a thermal sintering process at 350.degree.-400.degree. C. for their production, are relatively expensive to produce, and do not provide a desired degree of abrasion resistance.
U.S. Pat. No. 4,275,118 to Baney et al. describes a coating composition comprising an acidic dispersion of colloidal silica in an alcohol-water medium which produces a hard, abrasion-resistant coating.
U.S. Pat. No. 6,054,522 to Alain et al. discloses a protective coating which imparts non-stick, abrasion and hydrophobic properties with an inorganic network from metal alkoxides and an organic network from silanes via a sol-gel process. But this coating is very thin, which is not good as a protective coating, and also, has no color. These PTFE coatings have a finite life due to their organic makeup. Over time, these coatings dry out and lose their lubricity.
Various processes and materials have been proposed to create a non-stick coating. However, a problem persists regarding wear during use, in particular, scratching by cleaning agents.